As personal computers have become more sophisticated and widespread, electronic still cameras quickly have gained popularity as image input devices. The total pixel number of the solid-state imaging elements used in electronic still cameras has reached more than 1 million pixels, and recently, electronic still cameras provided with solid-state imaging elements having a total pixel number greater than 3 million pixels also have appeared on the market. Video cameras that are capable of shooting high-quality still images in addition to moving pictures also have been released on the market.
Although electronic still cameras come in many forms, one example is a compact type electronic still camera provided with a zoom lens having a ×2 to ×3 zoom ratio. Compact electronic still cameras must be easy to carry, and zoom lenses constituted by three groups have been proposed as zoom lenses that meet this requirement (for example, JP H11-52246A and JP H11-287953A). Zoom lenses constituted by three groups are made of a first lens group having a negative power, a second lens group having a positive power, and a third lens group having a positive power, arranged in that order from the object side to the image plane side. When zooming from the wide-angle end to the telescope end, the air gap between the first lens group and the second lens group is monotonically reduced and the air gap between the second lens group and the third lens group is monotonically increased. The third lens group is moved in the direction of the optical axis to carry out focus adjustment. Here, the third lens group is suited for autofocus because it is made of a single lens with a small outer diameter and can be driven at high speeds using a compact motor. Movement of the first lens group and the second lens group is carried out using cylindrical cams. Consequently, a collapsing configuration in which all three lens groups are drawn toward the solid-state imaging element using cylindrical cams when the zoom lens is not in use can be adopted. Also, if such a zoom lens is used in an electronic still camera, then the electronic still camera can be made thin in the depth direction when not in use.
With solid-state imaging elements, when the pixel number is increased but the picture size is kept the same, the pixel pitch becomes smaller, lowering the aperture ratio and the photosensitivity. Accordingly, by providing a miniature positive lens at each pixel of the solid-state imaging element, the effective aperture ratio is increased, preventing a drop in the photosensitivity. In this case, to let most of the light emitted from the miniature positive lenses arrive at the corresponding pixels, it is necessary to configure the zoom lens so that the principal rays that are incident on the pixels are substantially parallel to the optical axis. That is, there must be good telecentricity.
Solid-state imaging element performs spatial sampling due to their pixel structure, however, an optical low-pass filter generally is arranged between the zoom lens and the solid-state imaging element to remove the aliasing distortion that occurs at this time, removing high-frequency components from the image formed by the zoom lens. Optical low-pass filters generally are made of a quartz plate. Here, when natural light is incident on the quartz plate, the natural light is split into an ordinary ray and an extraordinary ray due to the birefringence of the quartz, and these are emitted parallel to one another.
Among video cameras, video cameras provided with a camera shake-correction function for correcting vibration in the captured image when the user's hand shakes have been released on the market. Many techniques have been proposed for camera shake correction, and a method for parallel displacement of a portion of the lens groups of the zoom lens in the direction perpendicular to the optical axis has been adopted (for example, JP 2000-298235A).
JP H11-52245A discloses a zoom lens made of a first lens group having a negative power, a second lens group having a positive power, a third lens group having a positive power, and a fourth lens group having a positive power or a negative power, arranged in that order from the object side to the image plane side, wherein correction of camera shake is carried out by parallel displacement of the third lens group in the direction perpendicular to the optical axis. This publication also discloses that decentered curvature of field and decentered coma aberration when the third lens group is parallel displaced can be corrected favorably.
Regarding the design of a zoom lens provided with a camera shake-correction function, Kenichi KIMURA et. al. in “Aberration Theory Applications in Anti-Vibration Optical Systems”. (19th Optics Symposium Proceedings, page 47 to 50, 1994) have proposed a method for using decentered third-order aberration. Also, Yoshiya MATSUI in “Theory of Decentered Optical System and Its Application” (Kogaku, Volume 24 Issue 12, page 708 to 712, December 1995) present a third-order aberration theory of decentered optical system .
To provide the images captured by an electronic still camera with high resolution, its zoom lens must have high resolution. However, the astigmatism of the zoom lens disclosed in the aforementioned JP H11-287953A is insufficient for correcting the curvature of field, and thus there is the problem that the overall image cannot be made high resolution.
To make an electronic still camera thin in the depth direction when not in use, it is possible to adopt a configuration in which the zoom lens collapses, however, to shorten the overall optical length (the distance from the object side end of the zoom lens to the light-receiving surface of the solid-state imaging element) when a three-group zoom lens is collapsed, it is necessary to shorten the overall length of each of the lens groups. However, with the zoom lens disclosed in the aforementioned JP H11-52246A, since the second lens group includes a long air gap or a lens with a thick center portion, there is the problem that the overall length of the second lens group is long and even if a collapsing configuration is adopted, the overall optical length when collapsed is not very short. Also, a moving lens barrel that moves during zooming and a stationary lens barrel that supports the moving lens barrel are required for a collapsing configuration, however, if the overall optical length during use is significantly longer than the overall optical length when collapsed, then the stationary lens barrel cannot stably support the moving lens barrel, and thus there is the problem that a portion of the lens groups becomes decentered, leading to a drop in the image-formation properties of the captured image.
The zoom lens disclosed in JP H11-52245A is made of ten or eleven lenses, and since the number of lenses is large, there is the problem of increased costs. Another problem with the zoom lens disclosed in this publication is that it has poor telecentricity.
Because the number of pixels of solid-state imaging elements used in electronic still cameras is much larger than the number of pixels of solid-state imaging elements used in conventional video cameras, when a video camera zoom lens provided with a camera shake-correction function is used in an electronic still camera, the image-formation properties at the peripheral portion of image in a state where camera shake is corrected are poorer than the image-formation properties in a state where camera shake has not been corrected (in the standard state).
If camera shake correction is performed by parallel displacement of a portion of the lens groups in the direction perpendicular to the optical axis, then to obtain good image-formation properties when camera shake is corrected, it is conceivably possible to reduce as much as possible the various aberrations in the lens groups that is parallel displaced. To do this, however, a large number of lenses must be combined, making the lens group that is parallel displaced heavy. When the lens group that is parallel displaced is heavy, an actuator with a large drive force and large external dimensions must be used to secure the required response speed. However, when the actuator is large, it is difficult to achieve an electronic still camera that is compact.